1. Technical Field
The present disclosure relates to a power bipolar structure.
More specifically, the disclosure relates to a power bipolar structure including at least one power bipolar transistor having a finger structure coupled to at least one driving block.
2. Description of the Related Art
As it is well known, bipolar transistors are subjected to the phenomenon known as second breakdown that causes their breakdown or failure due to local increases of temperature, up to melting at high temperatures. This phenomenon of the second breakdown is one of the most serious problems that limits the use of bipolar transistors, for example in power applications.
It is suitable to note that the reason of the failure of a power bipolar transistor at high temperatures is substantially linked to the negative drift of the base-emitter voltage Vbe of the transistor itself, which increases with temperature the direct current of its collector terminal, for a value equal to about 8% per 170 degrees C.
Further to this increase of temperatures, it occurs in fact that hot spots are created on the surface of the bipolar transistor, due to the positive feedback of its base-emitter voltage Vbe. Short-circuits are also created in correspondence with the base-emitter junction with local formation of melting spots between metallic layers, in particular aluminum Al, and semiconductor layers, in particular silicon Si, in the case of these materials, starting from 577 degrees C.
Moreover, for ensuring suitable voltage and current conditions in which a power bipolar transistor may operate without damaging itself, commonly indicated as safe operating area or SOA, the layout of such a power bipolar transistor is established taking also into account the current performance degradation in case of high values of the collector-emitter voltage Vce, which however further increases the final size of the transistor itself.
Different solutions have been proposed for reducing the problems linked to the phenomenon of second breakdown of power bipolar transistors. For example, it is possible to make reference to the article to Flavio F. Villa entitled: “Improved Second Breakdown of Integrated Bipolar Power Transistors”, IEEE Trans. Electron Devices, vol. ED-33, pp 1971-1976, December 1986.
In particular, a first branch of known solutions aims at ensuring a uniform current flow at the base-emitter junction of a power bipolar transistor, so as to limit in particular the generation of hot spots in this junction, limiting in this way the risk of a melting between silicon and metallization layers.
Normally, the uniformity of the current flow is obtained by trying to balance the power bipolar transistor with a suitable layout and with a finger structure, in particular for its emitter terminals, aimed at trying to avoid the problem of local hot spots, this finger structure being distributed so as to occupy a wide silicon surface.
A first known solution, schematically shown in FIG. 1, uses such a finger structure and sections the power bipolar transistor and its driver so that each finger of the power bipolar transistor has its dedicated driver.
In particular, FIG. 1 shows a power bipolar structure, globally indicated with 10. The power bipolar structure 10 comprises a power bipolar transistor with a finger structure, schematically shown as a power section 11 comprising a plurality of power transistors, Tp1 . . . Tpn. The power bipolar structure 10 also comprises a driving section 12 connected to the power section 11.
More in particular, the power transistors, Tp1 . . . Tpn, of the power section 11, corresponding to different fingers, in particular emitter ones, of a power bipolar transistor, have respective first terminals, in particular collector ones, connected to each other and to a first terminal of the power bipolar structure 10, in particular a collector terminal C, second terminals, in particular emitter ones, connected to each other and to a second terminal of the power bipolar structure 10, in particular an emitter terminal E, as well as third terminals, in particular base ones, connected to the driving section 12.
Similarly, the driving section 12 comprises a plurality of driving transistors, Td1 . . . Tdn, in a number corresponding to the plurality of power transistors, Td1 . . . Tdn, of the power section 11, i.e., to the number of emitter fingers of the power bipolar transistor comprised in the power bipolar structure 10. The driving transistors, Td1 . . . Tdn, have respective first terminals, in particular emitter ones, connected to the collector terminal C of the power bipolar structure 10, second terminals, in particular collector ones, connected to respective base terminals of the power transistors, Tp1 . . . Tpn, of the power section 11, as well as third terminals, in particular base terminals, connected to each other and to a third terminal, in particular a base terminal B of the power bipolar structure 10.
The type of layout of the power bipolar structure 10 allows to obtain a remarkable improvement of the safe operating area SOA with respect to a standard layout.
For making a power bipolar structure stronger with respect to the second breakdown current, usually indicated as Isb, it is also known to use ballast resistances connected to the base and/or emitter terminals of the structure, as schematically shown in FIG. 2A.
In particular, a bipolar structure 20 comprises at least one power bipolar transistor T1, in particular with a finger structure or a multiemitter, and a driving bipolar transistor T2 inserted, in parallel to each other, between a first terminal, in particular a collector terminal C, and a second terminal, in particular an emitter terminal E of the bipolar structure 20. The bipolar structure 20 also comprises a first ballast resistor Rb connected to the base terminals of the transistors T1 and T2, in turn connected to a third terminal, in particular a base terminal B of the bipolar structure 20. The bipolar structure 20 also comprises further ballast resistors, Re1 and Re2, connected to respective emitter terminals of the multiemitter power bipolar transistor T1.
It is suitable to underline that the presence of the ballast resistors, connected to the base and/or emitter terminals of the bipolar structure, makes this bipolar structure stronger against the breakdown phenomenon due to the negative feedback that these resistances create when the intrinsic current of the bipolar structure increases. Thanks to this feedback, the current in the base terminal B of the bipolar structure 20 is reduced, which reduces the current in the collector terminal C.
A first example of layout for a bipolar structure 20 is shown for example in FIG. 2B, globally indicated with 200.
In particular, in the layout 200 at least the power bipolar transistor T1 and the driving bipolar transistor T2, base contacts 201 and emitter contacts 202, as well as at least one ballast resistance indicated with 203 are present.
An alternative embodiment that is widely used as layout for a bipolar structure is schematically shown in FIG. 2C, globally indicated with 200′. Elements corresponding to the embodiment shown in FIG. 2B have been given the same reference numbers by way of illustration.
In particular, the layout 200′ comprises a first layer 210, in particular a base one, wherein the base contacts 201 are realized as well as a second layer 220, in particular an emitter one, wherein the emitter contacts 202 are realized and at least one ballast resistance, in particular a base resistance indicated with 203.
Although advantageous under several viewpoints, also these known solutions do not allow a correct control of the second breakdown current of a power bipolar transistor and do not allow thus to have a uniform current flow at its base-emitter junction so as to limit the generation of local hot spots and allow to use such a bipolar structure also in power applications.